Double-stranded breaks (DSBs) in DNA occur in eukaryotes during normal DNA replication, recombination, and DNA damage, but must be repaired to maintain cellular viability. The RAD51 gene was discovered to play a critical role in double strand break repair in the budding yeast S. cerevisiae. Its close homologs and paralogs may also have important functions on DNA repair and chromosome stability in multicellular organisms. The experimental advantages of the zebrafish (Danio rerio) allow us to test two important hypotheses regarding the function of the RAD51 family members in vertebrates: Hypothesis 1: RAD51 and several RAD51 paralogs are expressed in proliferating cells. We will test this hypothesis using whole mount in situ hybridization in various stages of embryo and larval development. Hypothesis 2: Normal levels of expression of at least some RAD51 family members are important for embryonic development and/or genomic instability. We will test this hypothesis using morpholino knockdowns of these genes. These experiments will pave the way for more extensive studies in the near future.